Goodbye in korean
Do you say mostly 안녕히 계세요 or 안녕히 가세요?

Question

Answer

안녕히 계세요 when YOU LEAVE (계시다 = 있다 = to stay → "Stay in peace")
안녕히 가세요 when THE OTHER PERSON LEAVES (가다 = to go → "Go in peace")

Answer

yes, usually like Hyejeong said. 
Guests say 안녕히 계세요 when they leave. 
Hosts say 안녕히 가세요 when guests leave.

Happy new year

Answer

It's not a matter of what's more commonly used - they're both used equally. It just depends on the situation.